IT has been known at least since the work of Hudson1 that reefs in the British Dinantian occur in the boundary regions between subsiding basins and rigid blocks, and that they are found at two principal horizons—in the basal Visean C2 zone (of northern England) and in the middle Visean D1 zone. The Lower Carboniferous sea was transgressive and, because the basin was expanding, the lower C2 reefs are found nearer the middle of the basin than the D1 reefs. Thus at the northern edge of the basin, the C2 reefs of Clitheroe, Whitewell and so on are situated farther south than the D1 reefs of Settle, Malham and so on. A similar situation is found in the Dublin district, where the C2 Feltrim and associated reefs occur farther south than the D1 reef of Curkeen Hill.